Professor Tobin Grant, department chair of Political Science at Southern Illinois University, pointed to other trustworthy polls, such as one from the General Social Survey. Those polls show that the white evangelical population has not declined, but rather remained stable over the past decade.

"The decline that they (PRRI) show in their survey doesn't match what we see in other surveys that are of higher quality and are seen as more accurate," Grant told The Christian Post, citing a General Social Survey graph of "white (not latino) 'Born Again'" Christians, which shows a mostly flat line from 2004 to 2016 at around 20 percent.

PRRI's study was released last week and revealed that the percentage of white evangelical Protestants dropped from 23 percent in 2006 to 17 percent of Americans in 2016. The study was based on interviews with more than 101,000 Americans from all 50 states.

Andrew R. Lewis, assistant professor at the University of Cincinnati, and Ryan P. Burge, who teaches at Eastern Illinois University in Charleston, also took issue with PRRI's report. They decided to compare PRRI's results with findings from both the GSS and the Cooperative Congressional Election Study.

"Both have the advantage of being large, national samples, and both include two different approaches to measuring evangelicals — affiliation with specific denominations and self-identification with evangelicalism (which PRRI used)," they said.

They also noted that the other surveys use two different methods — the GSS is conducted face-to-face, while the CCES is conducted online.

"Essentially, comparing the GSS and CCES data with the PRRI results using both measurement strategies allows us to fully assess if there is decline," they said.

Here is what the surveys showed: "In the GSS, the percentage of evangelicals by affiliation declines slightly (2-3 percent), but evangelicals by self-identification (the approach PRRI used) remain constant. In the CCES, evangelicals by self-identification actually increase from the 2008 baseline, while evangelicals by affiliation remain relatively constant. In fact, the only statistically significant change in all the data is an increase in self-identified evangelicals in the CCES from 2008 to 2012 (and nearly 2016)."

Lewis and Burge believe PRRI accurately captured the percentage of white evangelicals in 2016 (at 17 percent), considering the GSS has them between 18 and 19 percent and CCES has them between 16 and 18 percent.

The problem lies in the reported percentage of evangelicals in 2006. PRRI reported it as 23 percent, based on Pew reports, while the GSS found it to be about 19-20 percent in 2006 and CCES reported it as between 14-18 percent in 2008. Hence, PRRI came to the conclusion that the percentage of white evangelicals "dropped substantially" over the past decade while the other surveys showed consistency.

Grant told CP that "no survey is perfect," but noted that the GSS is "considered the gold standard on how to do a survey on these types of questions."

Speaking about PRRI's methodology, he pointed out that the researchers "did not do a survey of 100,000 people."

"What they do is a little survey every week, and then add it all together as if it's one big survey," the political science professor said.

"And where that's important is that rather than coming up with a list of numbers to call and people to contact, and then really making the effort to make sure you include as many of those people as possible, their surveys are done between Wednesdays and Sundays, until they get 1,000 people, and then they are done."

"What happens in those types of surveys," he continued, "is that they are not as representative and of the same quality [as] surveys where they really make an effort over a long period of time to contact people."

PRRI's methodology section for the 2016 American Values Atlas notes that the 101,438 telephone interviews were conducted between January 6, 2016 and January 10, 2017.

"Throughout 2016, at least 1,000 interviews were completed each week, with about 600 interviews conducted among respondents on their cell phones. Each week, interviewing occurred over a five-day period, from Wednesday through Sunday or from Thursday through Monday," the study states.

Grant clarified that he is "not questioning the integrity of the people who are involved" with the PRRI survey, but suggested that they are "overselling" their findings.

He said that a smaller survey of 2,000 people that really is representative of the people would be better than one with 100,000 people that is not so representative.

PRRI CEO Robert P. Jones told CP in response that their latest findings on white evangelicals do corroborate with other reports on membership declines and pointed to the Pew Research Center as well as Southern Baptist membership reports.

"According to Pew, the percentage of white, non-Hispanic Protestants has dropped from 23 percent in 2006 to 17 percent in Pew's latest publicly available survey (October 2016). The 2006 number is the exact number PRRI used in our report, and the most recent Pew number is identical to PRRI's finding that the percentage of white evangelical Protestants has fallen to 16.8 percent," the PRRI CEO said.

As for possible reasons for the discrepancy with the GSS study, Jones said that the latter "has a much smaller sample size," and noted that it asks a slightly different question to identify evangelicals, namely whether they have had a born-again experience.

"These trends are also corroborated from other sources that point in the same direction. For example, the Southern Baptist Convention — the largest white evangelical Protestant denomination — has now posted a straight decade of membership declines," he continued.

"And if one looks at the generational differences among American adults, they reinforce the trend numbers. White evangelical Protestants comprise 26 percent of seniors but only eight percent of the youngest cohort of Americans (ages 18-29)," he added.

As CP reported in June, the SBC has lost hundreds of thousands of members in the past few years, with baptism numbers also falling.

SBC leadership has signaled its intent to reverse the decline, however, vowing "to pray for and invest in evangelism and discipleship efforts with college students and strengthen the relationship between parachurch campus ministries and local churches."